The present invention relates to an RGB encoder which receives a primary color video signal (that is, an RGB signal) and a composite synchronizing signal so as to produce a composite video signal, and more particularly, to an RGB encoder whose operation mode can be switched between the National Television System Committee (NTSC) system and the phase alternating line (PAL) system, in accordance with an NTSC/PAL select signal.
FIG. 1 is a schematic block diagram of a conventional RGB encoder for combined NTSC/PAL systems, which includes a Y-matrix processor 101, an R-Y processor 102, a B-Y processor 103, a burst-gate-pulse (BGP) generator 104, a sync separator 105, an R-Y signal modulator 106, a B-Y signal modulator 107, a phase shifter 108, an NTSC/PAL selector 109, an oscillator 110, an adder 111, a chroma buffer 112, a bandpass filter (BPF) 113, a sync adder 114, a delay element 115 and a mixer 116.
Referring to FIG. 1, Y-matrix processor 101 mixes the primary color signal which constitutes R, G, and B (red, green, and blue) signals produced from an electronic device such as camera tube, microcomputer, etc., in specific proportions, so as to produce a luminance signal (Y). Here, the mixing ratio of the R, G and B signals is controlled by controlling the ratios of the resistances of resistors which are connected to the respective RGB input terminals. The thus-produced luminance signal is summed with composite sync (Csync) pulses in sync adder 114. Sync separator 105 separates horizontal synchronizing pulses from the received composite sync pulses and then applies the separated horizontal synchronizing pulses to BGP generator 104. A burst-gate pulse is active during the transmission of the color burst signal in the composite video signals, that is, it is active during a predetermined time (about 0.5 .mu.s after the horizontal synchronizing pulse). R-Y processor 102 subtracts the luminance signal produced in Y-matrix processor 101 from the R signal of the received RGB signal, so as to produce a first color difference signal (R-Y), and B-Y processor 103 subtracts the luminance signal from the B signal, so as to produce a second color difference signal (B-Y) . Then, the R-Y and B-Y color difference signals are respectively modulated in R-Y modulator 106 and B-Y modulator 107. The carrier used in R-Y modulator 106 and B-Y modulator 107 is produced in oscillator 110 and then phase-shifted in phase shifter 108, in accordance with the NTSC/PAL select signal. First, for an NTSC system, the carrier used in R-Y signal modulator 106 has a 90.degree. phase difference with respect to that used in B-Y modulator 107. For PAL systems, the carriers used in R-Y modulator 106 also has a 90.degree. phase difference with respect to that used in B-Y signal modulator 107 but the carder used in R-Y modulator 106 is phase-shifted by 180.degree. every other horizontal scanning line. Accordingly, phase shifter 108 receives the signal produced from oscillator 110 and then, for NTSC systems, outputs the received signal and one phase-shifted by 90.degree., and for PAL systems, outputs the received signal and a signal which is phase-shifted by 90.degree. and 270.degree. alternately, every other horizontal scanning line. The modulated outputs of R-Y modulator 106 and B-Y modulator 107 are summed in adder 111 to produce a chrominance signal which is then applied to bandpass filter 113 via chroma buffer 112. Bandpass filter 113 removes the noise included in the chrominance signal applied via chroma buffer 112. The chrominance signal has an approximately 1 MHz bandwidth and its center frequency is the same as that of the color subcarrier. Bandpass filter 113 passes only the components of this frequency band, i.e., that of the chrominance signal, thereby attenuating the noise outside the passband. Here, the center frequency of the color subcarrier is about 3.58 MHz for NTSC systems and about 4.43 MHz for PAL systems. The conventional bandpass filter will be explained later referring to FIG. 2. Delay element 115 compensates for the time delay difference between the signal processing of the luminance signal and that of the chrominance signal. Here, the time delay of an NTSC system is different from that of a PAL system. Mixer 116 mixes the luminance signal applied from delay element 115 with the chrominance signal applied from bandpass filter 113, so as to produce a composite video signal.
In the conventional RGB encoder for combined NTSC/PAL systems constructed as above, the operation of delay element 115 and bandpass filter 113 in an NTSC mode should differ from that in a PAL mode. There are two conventional methods to accomplish this. In one conventional method, the delay element and the bandpass filter are both formed of discrete components, such that time constants of these circuits can be controlled. However, an RGB encoder having the discrete-circuit-type delay element and bandpass filter complicates the application circuit thereof, so as to be undesirable.
In the other conventional method, as shown in FIG. 2, two delay elements and two bandpass filters are formed separately: one delay element and one bandpass filter for the NTSC system and one delay element and one bandpass filter for PAL system, with only one pair being enabled in accordance with the NTSC/PAL select signal. Two delay elements and two bandpass filters can be integrated in a semiconductor circuit chip but the overall circuit structure thereof is very complicated due to critical circuit design requirements, which results in increased manufacturing cost. Also, power consumption is high when using such a structure.